Thin-film wiring substrates each including a thin-film wiring layer laminated on an upper surface of a ceramic substrate have been known as wiring substrates (space transformer substrates) for electrically connecting connection pads provided on a lower surface to external electric circuits, the connection pads being electrically connected to terminals on an upper surface to which semiconductor elements are connected. The thin-film wiring substrates are used as, for example, substrates for so-called probe cards adapted for electrically testing semiconductor elements. In testing semiconductor elements, the thin-film wiring substrates are pressed on the semiconductor elements with predetermined pressure.
The ceramic substrates each include a wiring conductor which is formed on the upper surface of an insulating base composed of an aluminum oxide sintered body or the like, and a connection pad for external connection which is formed on an outer surface such as a lower surface, the wiring conductor being electrically connected to the connection pad. The thin-film wiring layer is formed by, for example, depositing a thin-film conductor composed of a copper plating layer or the like on a surface of a resin insulating layer. In the thin-film conductor, a portion exposed in the uppermost surface of the thin-film wiring layer functions as a terminal to be connected to an electrode of a semiconductor element.
In recent years, a thin-film wiring substrate has been proposed in which a thin-film wiring layer and a ceramic substrate, which are separately formed, are bonded through a bonding layer composed of an adhesive resin material. This is based on consideration of productivity and practicability (so-called coping with wide-variety production) of thin-film wiring substrates. The bonding layer has a through conductor (via conductor) formed for electrically connecting a wiring conductor of the ceramic substrate to a thin-film conductor of the thin-film wiring layer.